In many situations it is useful for an amplifier to have two different gain settings. For example, if a variable gain amplifier (VGA) is used to drive an analog-to-digital converter (ADC or A/D converter), it may be useful to provide two different gain ranges: a high gain range for driving 10-bit converters, and a low gain range for driving 12-bit converters.
FIG. 1 illustrates such an arrangement, which is based on a VGA having a continuously interpolated attenuator. The input signal is applied to an attenuator network 102. Attenuated versions of the input signal are available at tap points along the attenuator. A steering circuit 104, typically based on a series of transconductance (gm) cells controlled by an interpolator, selects the signals from one or more tap points and feeds them to a fixed gain amplifier 106. By selecting various tap points along the attenuator and merging the signals from adjacent tap points, the steering circuit provides continuously variable gain control. A switched feedback network 108 for the fixed gain amplifier allows the user to switch between high and low gain ranges by opening or closing switch 110. This changes the bandwidth in a voltage feedback amplifier with fixed compensation.